Partak and Bandak are spawned by finding all the dying orcs in the towers around the keep and clicking them to put them out of their misery:

Tower Top

( -36, 10, 10 ) /waypoint -36.02, 10.00, 10.03

( 30, 10, -42 ) /waypoint 29.90, 10.00, -41.70

Tower Midlevel

( -24, 9, -37 ) /waypoint -24.01, 8.68, -37.28

( 26, 9, 59 ) /waypoint 26.09, 8.73, 58.90

Tower Bottom Floor

( -25, 0, 63 ) /waypoint -24.74, 0.00, 62.82

( 31, 0, 62 ) /waypoint 30.75, 0.00, 61.88

( 48, 0, 9 ) /waypoint 47.70, 0.00, 9.06

( -29, 0, -41 ) /waypoint -28.79, 0.00, -41.06

Roof of Courtyard Buildings

( -22, 5, 5 ) /waypoint -22.48, 4.85, 4.64

( 23, 5, 16 ) /waypoint 22.87, 4.79, 15.66

Partak and Bandak gain an incremental which, when it hits 20, lets them one-shot you. To avoid this, burn them down equally, switching back and forth between the two to keep their health approximately even are a tank and spank that gain increments when they take damage.